One conventional data storage system includes a storage processor, an array of magnetic disk drives and a backup power supply. The storage processor carries out a variety of data storage operations on behalf of an external host device (or simply host). In particular, the storage processor temporarily caches host data within its storage cache and, at certain times, de-stages that cached data onto the array of magnetic disk drives. If the data storage system is set up so that it acknowledges write requests from the host once the data reaches the storage cache rather than once the data reaches the array of magnetic disk drives, the host will enjoy shorter transaction latency.
Some data storage systems employ backup power supplies to prevent the loss of data from the storage caches in the event of power failures. For example, suppose that such a data storage system loses its steady state source of electrical power (e.g., power from the street) during operation. In such a situation, a set of backup power supplies provides reserve power to the storage processor and to a persistent storage device (e.g., the array of magnetic disk drives) for a short period of time (e.g., 30 seconds). During this time, the storage processor writes the data from its storage cache onto the persistent storage device so that any data which has not yet been properly de-staged is not lost. Once power from the main power feed returns, the storage processor loads the data from the persistent storage device back into the storage cache. At this point, the data storage system is capable of continuing normal operation.
Rechargeable batteries depend on a number of known cell types, including Ni-Cad, Ni-MH, and Li-Ion cells. All these cells are known to those of skill in the art, as are some of their deficiencies. One of the known deficiencies of the above mentioned rechargeable battery cells is related to the fact that each battery has a finite life span that can be measured in terms of recharge cycles. The process of charging and discharging the cell damages the cell's charge storage capabilities, causing the stored potential, which is typically measured in mA-hours, to decrease over the life of the battery. As the ability to store charge decreases, so does the battery's utility. The life of the battery can be drastically curtailed by improperly charging, or over discharging the battery. As a result of these deficiencies, it can be useful to be able to determine the capacity of a battery both prior to and during the usage.
One technique for battery capacity reporting relies on the coulomb counter. The principle of operation involved in coulomb counting is computing the difference between the coulombs injected into a battery and the coulombs taken out of the battery. The capacity of the battery is then reported by comparing the coulomb count relative to a reference coulomb count value that corresponds to maximum battery capacity. For instance, if the coulomb count of a battery is half of the reference value, the battery capacity is reported to be 50 percent.
Other known existing techniques of battery capacity reporting are primarily based on measuring battery voltage. Batteries have known characteristic charge and discharge curves. When the battery is in a charging state, a charge curve corresponding to the charging state is utilized. When the battery is in a discharging state, a discharge curve corresponding to the discharging state is utilized. The charge and discharge curves are such that given a battery voltage value and a charge curve or a discharge curve, it is possible to obtain a corresponding capacity value from the curves.